Cercospora leaf spot (CLS) (Figure 1 and 2) is the most destructive foliar disease of sugarbeet in North Dakota and Minnesota.

Getting Prepared for MANAGING Cercospora Leaf Spot of Sugarbeet

Cercospora leaf spot (CLS) (Figure 1 and 2) is the most destructive foliar disease of sugarbeet in North Dakota and Minnesota. The causal agent of CLS is the fungus Cercospora beticola which does most damage in warm weather (day temperature of 80 to 90° F and night temperature above 60° F) and in the presence of moisture from rain or dew on the leaves. Since the fungus destroys the leaves, it adversely impacts photosynthesis resulting in reduced tonnage and 2-3 percent reduction in sugar concentration.

Growers should not allow the disease to become severe before initiating fungicide application since that will be a recipe for disaster. It is recommended that after row closure, fields should be scouted every 5 days so that the first application can be made at first symptoms. Symptoms typically appear first in fields close to waterways, shelterbelts, last year’s sugarbeet fields, and next to corn fields.

The best way to control CLS during the growing season is to apply fungicides in a timely manner. For ground application, apply fungicides in 15 to 20 gallons of water per acre at 75-100 psi pressure; aerial applicators should use 5 gallons of water per acre for best results. Use full rates of fungicides when they are used alone, and a minimum of 0.8 times the labeled rates of each fungicide when using mixtures.

Research shows that application of effective fungicides at first symptoms, and subsequent applications at least at 14 day intervals based on the presence of leaf spots and favorable environmental conditions (Daily Infection Values for two consecutive days of 7 or higher) consistently provided the most effective and economical control. In 2015, when environmental conditions were very favorable for CLS infection and development, research demonstrated that the timely use of different chemistries of fungicides in a rotation program provided effective disease control and did not impact yield nor quality. Mixtures of effective fungicides should be used especially in the first application to prevent the pathogen from increasing its population. Over the past decade, a mixture of Triphenyltin hydroxide (TPTH) (4L) and Topsin (F) fungicides as the first application followed by other chemistries in a rotation has consistently provided effective CLS control (Figure 3) compared to a non-treated check (Figure 4). Since NDSU’s laboratory data based on samples collected from growers’ fields indicate an increasing number of Cercospora beticola isolates with resistance to triazoles and QoI fungicides, the use of a Topsin + Tin mixture should help to control any overwintering CLS isolates which are fungicide resistant. The triazoles can then be used in rotation with the strobilurins (or QoI), with a fourth application, if necessary in a heavy disease year, in the form of TPTH. Please consult your agriculturists to determine if there are fungicide resistance issues in your township so that a more specific rotation can be developed for your field. In principle, most of the fungicides (TPTH, Topsin, Eminent, Minerva, Minerva Duo, Inspire XT, Proline, Topguard, Headline, Gem, and Priaxor) recommended will provide effective CLS control once they are used in a timely manner and in a rotation (individually or in mixtures) program.

Most fungicides in a liquid form, with the exception of Minerva Duo, can be mixed with glyphosate for control of CLS and weeds. It is recommended to do a compatibility test with each batch of mixtures to ensure that the mixture will not result in nozzle plugging. Research is ongoing to determine the utility of using mixing copper products with other recommended fungicides for controlling CLS and managing fungicide resistance.